This invention relates to an ignition system for an internal combustion engine, and more particularly to a capacitor discharge type ignition system for an internal combustion engine.
In general, an internal combustion engine tends to cause a concentration of fuel to be reduced at the time of quick acceleration of the engine as compared with at the time of its normal or steady operation, because the concentration is adjusted at each of engine speeds in view of steady operating conditions of the engine. In particular, an internal combustion engine used for a vehicle for motocross or a racing car is frequently subject to quick acceleration or quick deceleration. During a transient state such as quick acceleration, quick deceleration or the like, a concentration of fuel is considerably deviated from its ideal concentration.
As described above, a decrease in concentration of fuel causes incomplete combustion after ignition of the engine, resulting in the engine failing to exhibit a sufficient output. Thus, although, in a bench test, the engine exhibits a sufficient output over a range of from a high engine speed region to a high engine speed region, it fails to exhibit a sufficient output at the time of quick acceleration in a test which causes a vehicle to run while mounting the engine on the vehicle.
In particular, when a capacitor discharge type ignition system wherein charges in a capacitor for accumulating ignition energy are discharged to a primary winding of an ignition coil to induce a high voltage for ignition of the engine is used for an internal combustion engine, it is possible to obtain a secondary voltage rapidly rising. However, this fails to permit a duration of a discharge current of the capacitor to be increased, to thereby cause incomplete combustion when a concentration of fuel is reduced at the time of quick acceleration of the engine.
Recently, an internal combustion engine tends to employ, with an increase in tightening of control of exhaust gas, a lean combustion system which is an ignition system adapted to carry out combustion using fuel of a reduced concentration. For this purpose, it is required to construct the ignition system so as to permit a secondary voltage across the ignition coil to rapidly rise and a duration of a secondary discharge current (spark current) to be increased.
In view of the above, such an ignition system as disclosed in Japanese Patent Publication No. 452/1978 (53-452) was proposed, which comprises a combination of a capacitor discharge type ignition system constructed so as to permit a duration of a secondary discharge current to be reduced while rendering rising of a secondary voltage rapid and a battery interruption type ignition system constructed so as to permit a duration of a secondary discharge current to be increased while rendering rising of a secondary voltage slow and adapted to interrupt a current fed from a battery to a primary winding of an ignition coil to provide a high voltage. In the proposed ignition system in which the capacitor discharge type ignition system and the battery interruption type ignition system are combined, it is required to increase the number of turns of a primary winding of an ignition coil to a level as large as 200 to 300. This requires to increase a size of a core, leading to large-sizing of the ignition coil.
In order to avoid the problem, a capacitor discharge type ignition system was then proposed which is constructed in such a manner as taught in Japanese Patent Application Laid-Open Publication No. 204968/1985 (60-204968). More particularly, the ignition system proposed includes two sets of capacitor discharge circuits each comprising a capacitor for accumulating ignition energy therein and a discharge switch adapted to discharge charges in the capacitor to a primary winding of an ignition coil and is adapted to carry out dual ignition.
In the ignition system proposed, discharge of one of the capacitors takes place at a normal ignition position, to thereby cause a first ignition operation to be carried out and then discharge of the other capacitor is attained at a position slightly delayed on the basis of the normal ignition position, leading to a second ignition operation. This results in increasing an apparent duration of discharge and preventing incomplete combustion due to a decrease in concentration of fuel.
The conventional capacitor discharge type ignition system intended to carry out dual ignition requires two sets of combinations of capacitors and switches for discharge of the capacitors, resulting in being complicated in structure.
Also, in the ignition system, the two ignition energy accumulating capacitors must be charged, so that it is required to increase a capacity of a power supply used for charging the capacitors. Unfortunately, this causes large-sizing of the system conjointly with the fact that two capacitors and two switches are required.
Techniques for carrying out multiple ignition in a capacitor discharge type ignition system for an internal combustion engine are disclosed in U.S. Pat. Nos. 3,718,125 and 4,149,508 as well.